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Chapter 12 – Neural Tissue!
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Chapter 12!Neural tissue! pp. 386–398; 416–424 !
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Nervous System Overview!
Nervous system!• Provides swift, brief responses to stimuli!
Endocrine system!• Adjusts metabolic operations and directs
long-term changes!Nervous system includes!• All the neural tissue of the body!• Basic unit = neuron!
Chapter 12 – Neural Tissue!
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SECTION 12-1 !The nervous system has anatomical and functional divisions!
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Divisions of the Nervous System!
CNS (Central Nervous system)!• Brain and spinal cord!
PNS (Peripheral Nervous system)!• Neural tissue outside CNS!• Afferent division brings sensory
information from receptors!• Efferent division carries motor commands
to effectors!a. Somatic NS: skeletal muscle!b. Autonomic NS: smooth, cardiac muscle,
glands!
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SECTION 12-2 !Neurons are nerve cells specialized for intercellular communication!
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Schematic Neuron!
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Neuron Anatomy Figure 12-2!
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Parts of a Neuron!
Cell body (soma)!Cell membrane!• Has different types of membrane channels!• Transmits graded potentials, not action
potentials!Cytoplasm!• Perikaryon = cytoplasm surrounding nucleus!• Neurofilaments, neurotubules = cytoskeleton!• Nissl bodies (substance)!
RER and free ribosomes → Gray matter!
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Parts of a Neuron (2 of 3)!Dendrites• Receive information from other neurons!• Carry information towards cell body!• Transmit graded potentials, not action potentials!• Most have very few voltage-gated Na+ channels
Axon• Axolemma, axoplasm!• Connects to soma at axon hillock!• First part = initial segment!• Initial segment generates action potentials!• Has voltage-gated Na+ channels
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Parts of a Neuron (3 of 3)!
Collaterals• Major branches of an axon!
Telodendria• Small branches at the end of an axon!
Synaptic terminals - ends of the telodendria!• a.k.a. synaptic end bulbs, boutons, synaptic
knobs!• Store neurotransmitter in synaptic vesicles!• Release neurotransmitter in response to
electrical activity!
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Axoplasmic Transport (1 of 2) kinesin movie!
A. “Slow stream” transport = axoplasmic flow• New cytoplasm (axoplasm) being produced!
Moves materials 1 - 5 mm per DAYB. “Fast stream” transport = fast axonal transport
Moves materials 5 - 10 mm per HOUR• Movement of:!
Neurotransmitters, enzymes, organelles!• Mechanism:!
“Molecular motors” move on “tracks”!Motors = kinesin, dynein (use ATP)!Tracks = neurotubules (microtubules)!
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Fast Axonal Transport (2 of 2)!
C. Directionality!• Anterograde (kinesins)!
Movement away from cell body!• Retrograde (dynein)!
Movement toward cell body!e.g. Metabolites, other chemicals!
May affect gene activity!e.g. Rabies virus, herpes virus, !
tetanus toxin!
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Based on number of processes attached to cell body!
Anaxonic!• Axon and dendrites not
structurally different!• Poorly understood!• Brain, special sense
organs!
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A. Structural Classification of Neurons!
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Unipolar Neuron!
Unipolar - one process!• A.K.A. pseudounipolar!• Sensory neurons!• Cell body in dorsal root ganglion!• Up to 1 m long!
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Bipolar Neuron!
Bipolar - two short processes!• Relatively rare!• Eye, ear, nose!• Special sensory !
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Multipolar Neuron!
Multipolar!• More than two processes!• Most common type!• e.g. motor neurons!
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Sensory = afferent!• Carry information towards CNS!
!Motor = efferent!• Carry information away from CNS!
!Interneuron = association = internuncial!• Do not enter or leave CNS!
(exception: some are in autonomic ganglia)!
B. Functional Classification of Neurons!
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Receptor Types!
Interoceptors!• Sense internal environment!• e.g., body temp., heart rate, blood pressure,
etc.!Exteroceptors!• Sense external environment!• e.g., ambient temp., light, touch, sound, etc.!
!Proprioceptors!• Sense position of muscles and joint!• i.e., body position!
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SECTION 12-3 !CNS and PNS neuroglia support and
protect neurons!
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Introduction to Neuroglia Figure 12-5!
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CNS Glial Cells Figure 12-6!
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1. Ependymal cells!• Epithelial cells!• Have cilia or microvilli!• Contact other glial cells!
Functions:!• Assist in cerebrospinal fluid (CSF) production!• Assist in CSF circulation!• Monitor conditions in CSF!• Form blood-CSF barrier at choroid plexuses• May be able to differentiate into neurons!!!
CNS Glial Cells – Ependymal Cells!
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CNS Glial Cells – Astrocytes (1 of 2)!
2. Astrocytes - “star cells”!A. Maintain blood-brain barrier!• Affect capillary permeability!
B. Structural support for neurons!C. Repair damage → scar!D. In embryo, direct growth and
interconnections of developing neurons!
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CNS Glial Cells – Astrocytes (2 of 2)!
E. Control internal environment!• Regulate K+, Na+, etc.!• “Feed” neurons nutrients, ions, gases!• Regulate capillary blood flow!• Absorb and recycle neurotransmitters!• Modify synaptic activity!• Inhibit axonal growth!
F. May be involved in AP conduction by neurons!FYI: http://www.jneurosci.org/content/35/31/11105!Courtney Sobieski, Xiaoping Jiang, Devon C. Crawford and Steven Mennerick, 2015!
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CNS Glial Cells – Oligodendrocytes!
3. Oligodendrocytes = “few branches” compared to astrocytes!A. Most processes contact neuron cell bodies!• Function of these is currently unclear!
B. Myelinate CNS axons!• More than one at a time (in contrast to
Schwann cells)!C. Provide structural support!
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CNS Glial Cells – Microglia!
4. Microglia - “small”!• Derived from monocyte/macrophage stem
cells!• Can move through !
neural tissue!• Phagocytize:!
Cell debris!Wastes!Pathogens!
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1. Satellite cells (amphicytes)!• Found in peripheral ganglia!• Ganglion = a collection of
neuron cell bodies outside of the CNS!
• Regulate internal environment (like astrocytes in CNS)
PNS Glial Cells – Satellite Cells!
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PNS Glial Cells - Schwann cells!
2. Schwann cells (neurilemmocytes)!• Myelinate peripheral axons
(myelin sheath)!• “Enclose” unmyelinated
peripheral axons!• Myelin sheath functions in axon
repair and speed of action potential conduction
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Schwann Cells and Myelination – 1 Figure 12-7c!
A Schwann cell wraps itself around an axon, squeezing Schwann cell cytoplasm to the outermost end of the cell. This leaves many layers of Schwann cell membrane wrapped around the axon.!
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Schwann Cells and Myelination – 2 Figure 12-6!Myelinated! Unmyelinated!
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Schwann cell!
Oligodendrocyte!
Myelinated!PNS!
Unmyelinated!
Myelinated !CNS!
Myelination – Schwann cells/oligodendrocytes!
Page 96 in Leboffe!
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Neural Response to Injury!
For repair to occur, neuron cell body must remain alive!• Most mature neurons thought to be in G0!
Exceptions:!• Active stem cells present in olfactory
epithelium and hippocampus (brain region)!!CNS repair of axons very limited:!
1. Injury would destroy many axons at once!2. Astrocytes produce scar - block axon
re-growth!3. Astrocytes release axon growth inhibitors!
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PNS Repair of Axons!
Repair involves Schwann cells!Axon injury → Wallerian degeneration!
1. Axon distal to injury site degenerates!2. Schwann cells divide, form tube!3. Macrophages engulf debris!4. Axon forms bud, grows into tube!5. Schwann cells myelinate axon!
!See Figure 12-8!
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PNS Axon Regeneration Figure 12-8!
Assumes cell body is still alive!
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SECTION 12-6 !Axon diameter, in addition to myelin, affects propagation speed!
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Action Potential Conduction Velocity!
Action potential velocity is influenced by:!1. Axon diameter!• ↑ Fiber diameter → ↑ velocity!
2. Presence of electrical insulation (myelin)!• ↑ Electrical insulation → ↑ velocity!
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Type A fibers!• Largest!• Myelinated!• Velocity to 140 m/sec (300 mph!!)!
Type B fibers!• Medium size!• Myelinated!• Velocity about 18 m/sec (40 mph)!
Type C fibers!• Smallest!• Unmyelinated!• Velocity about 1 m/sec (2 mph)!
Axon Classification - Speed of Conduction!
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Myelination and Saltatory Conduction!
Your drawing here…!
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Refractory Periods!
Refractory period!• Period of time between initiation of AP and
restoration of normal resting potential!• Membrane does not respond normally to
stimulation!A. Absolute refractory period!• All Na+ gates are open or inactivated
(locked)!• No amount of stimulation will cause another
AP!!
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Refractory Periods - 2!
B. Relative refractory period!A greater than normal amount of stimulation is required to begin another AP!• Na+ gates closed, but not inactivated.!• Voltage-gated K+ channels open and/or!• Membrane hyperpolarized!
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Refractory Periods See Figure 12-14!
Na+ channels closed, but not inactivated.!
OR !All Na+ channels open.!
All Na+ channels inactivated.!
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SECTION 12-7 !At synapses, communication occurs among neurons or between neurons and other cells!
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Action potential travels along an axon!Information passes from presynaptic neuron to
postsynaptic cell!
Information Flow!
Direction of information flow
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Information Flow Is Unidirectional!
Information flow is unidirectional.!1. Refractory periods do not allow impulse to
go back where it just came from.!2. Neurotransmitter only released by one end
of the neuron!!
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1. Electrical!• Pre- and postsynaptic membranes
fused at gap junctions• Relatively rare, but more are currently
being found in certain brain regions! E.g. Some hormone-secreting cells
in hypothalamus; all cells release hormone at same time!
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Types of Synapses – 1!
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Types of Synapses – 2!
2. Chemical!• Review cholinergic synapse from 251!• Note that one “dose” of neurotransmitter is
usually not sufficient to cause a new action potential in a postsynaptic neuron.!
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Review of Cholinergic Synapse Figure 12-16!
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Properties of Chemical Synapses!
A. Synaptic delay: 0.2—0.5 msec delay at synapse!• Mostly due to time for Ca2+ influx into bouton and
neurotransmitter release!I.e. not the time for neurotransmitter diffusion!
• Is a short delay, but can add up!ü Reflex arcs have few synapses → FAST!
B. Synaptic fatigue!• Possible to run out of neurotransmitter!
(But not a factor in muscle fatigue!)!
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Properties of Chemical Synapses - 2!
C. Drug effects on synaptic transmission!1. Block voltage-gated channels - TTX!2. Demyelinate axons - lead, arsenic!3. ↑ neurotransmitter release - spider venom!4. Block binding sites - atropine, curare!5. Stimulate receptors/mimic - nicotine!6. ↓ transmitter inactivation - anti-AChE, nerve
gas!7. Block transmitter release - C. botulinum toxin!8. Depolarize axon hillock - caffeine!
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Some Drug Effects !
From 8th!
edition of text!
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SECTION 12-8 !Neurotransmitters and neuromodulators have various functions!
Free children’s dental health day February 4, 2017!Ages 1 – 18!!Cleaning, Dr. exam, x-rays, sealants, fluoride!!Call 360-992-2158 for an appointment.!
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Mechanisms of action!• Depend upon transmitter binding to different
types of receptors!A. Ionotropic receptors - DIRECT EFFECTS!• Receptor is an ion channel• Binding of transmitter directly alters ion
channel!
Neurotransmitters/Receptors Figure 12-17a!
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B. Metabotropic Receptors (1 of 2)!
B. Metabotropic receptors - INDIRECT EFFECTS!• Receptor is not an ion channel!• Binding of transmitter:!
a. Indirectly causes ion channel to open!- AND/OR -!
b. Changes cellular metabolism!• Involves G-protein!• Usually involves a second messenger!
(e.g. cAMP)!
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Metabotropic Receptors (2 of 2)!
Mechanism of action!1. Neurotransmitter binds to receptor!2. G-protein activated!3. GTP and part of G-protein separate!
a. Subunit opens ion channel - OR -!b. Subunit activates second messenger!
Second messenger (e.g. cAMP):!• Opens ion channel - OR –!• Activates intracellular enzymes that
change cellular metabolism!
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Metabotropic Receptor Actions Figure 12-17b!
Indirect effects via G protein!
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C. Indirect Effects - No Membrane Receptor
e.g. Nitric oxide (NO) and carbon monoxide (CO)!• Lipid-soluble gases!• Not stored - synthesized as needed!• Easily enter cell!• Bind to enzymes!• Second messenger produced!
Open ion channels - AND/OR -!Change cell metabolism!
!• E.g. NO involved in erectile function!
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Indirect Effects of NO and CO Figure 12-17c!
No membrane receptor
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Neurotransmitter Types (1 of 2)!
1. ACh!2. Biogenic amines!• EPI, NE, dopamine, serotonin, histamine!
3. Amino acids!• Glutamate, aspartate, glycine, GABA!
4. Neuropeptides!• Enkephalins, endorphins, substance P!
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Neurotransmitter Types (2 of 2)!5. Purines!• ATP, Adenosine, GTP!
6. Gases!• Nitric oxide, carbon monoxide!
7. Hormones - ADH, insulin, glucagon!!
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Examples of Neurotransmitter Actions!
Excitatory - direct effect!• Open channels for Na+ and close K+ - e.g.
ACh!• Open Ca2+ channels - e.g. Glutamate!
Inhibitory - direct effect!• Open Cl- channels - e.g. Glycine, GABA!• Open K+ channels - e.g. GABA!• Block Ca2+ channels - e.g. GABA!
Whether direct or indirect, excitatory or inhibitory, the effect depends upon transmitter and receptor type.
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SECTION 12-9 !Information processing!
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Information Processing!
• Post-synaptic cell receives many inputs!• Effect of presynaptic cell activity on postsynaptic
cell’s membrane = postsynaptic potential!
Direction of information flow
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Postsynaptic potential are graded potentials and can be:!
!Excitatory - EPSP!• Postsynaptic cell closer to threshold!• More likely to fire action potential!
!Inhibitory - IPSP!• Postsynaptic cell further from threshold!• Less likely to fire action potential !
Postsynaptic Potentials See Figure 12-19!
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EPSPs and IPSPs: Effects/Interactions Figure 12-19!
Recording from a postsynaptic cell!
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Summation!
Postsynaptic potentials are added together!• If initial segment reaches threshold → action
potential!• If initial segment does not reach threshold →
no action potential!!Temporal summation!• Single synapse fires repeatedly!
Spatial summation!• Different synapses fire at same time!
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Summation Scheme!
Temporal summation:!• Neuron A fires repeatedly!
Spatial summation:!• Neurons A and B fire at the same time!
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EPSPs and IPSPs: Effects/Interactions Figure 12-19!
Recording from a postsynaptic cell!
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May occur at axoaxonal synapses!A. GABA released onto presynaptic axon!B. Inhibits opening of voltage-gated Ca2+
channels!• ↓ Ca2+ entry into bouton!• ↓ neurotransmitter release!• ↓ effect on postsynaptic cell!
Presynaptic Inhibition Figure 12-20a!
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Presynaptic Facilitation Figure 12-20b!
May occur at axoaxonal synapses!A. Serotonin released onto presynaptic axon!B. Inhibits closing of voltage-gated Ca2+ channels!• ↑ Ca2+ entry into bouton!• ↑ neurotransmitter release!• ↑ effect on postsynaptic cell!
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Information is interpreted on the basis of action potential frequency (AP/second)!• Few impulses/sec - sensed as light touch!• Many impulses/sec - sensed as pain!
!Initial segment can fire second AP if axon hillock
remains above threshold:!• Fires when absolute refractory period is over !
- and -!• Fires if enough depolarization exists during
relative refractory period!
Rate of Generation of Action Potentials!
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Environmental Factors Affect Nerve Function!
1. Hydrogen ion concentration (pH)!H+ modulates (reduces) ion channel opening!• Alkalosis - increased excitability!
(less interference with Na+ gate opening)!• Acidosis - decreased excitability!
(more interference)!
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Effects of [Ca2+] in ECF!
2. Ionic composition of ECF![Na+], [K+], [Ca2+] also very important!!e.g. Effects of [Ca2+]!
Ca2+ modulates Na+ channel opening!• Hypocalcemia → seizures!
Causes ↑ membrane permeability to Na+!Promotes depolarization, excitability!
• Hypercalcemia - CNS depression!
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Pressure and Temperature Effects!
3. Pressure!↑ Pressure → ↓ blood flow → ↓ neurotransmission!
!4. Temperature!• Increased temperature → increased
excitability!• Decreased temperature → decreased
excitability!(ice pack on injury)!
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Metabolic Considerations!
Brain = 2% of body weight!Brain uses 18% of resting oxygen consumption!• Synthesis, release, recycling of
neurotransmitters!• Maintain resting potential (Na+/K+ pumps)!• Recovery of resting potential after action
potential!ü More APs → need more Na+/K+ pump
activity to restore resting conditions!• Axonal transport of transmitters, organelles,
etc.!